The present invention relates to an electrically powered steering for automobiles and the like, in particular to the improvement in steering feeling in case of failure.
In FIG. 10 shows a typical electrically powered steering system. In FIG. 10, reference numeral 100 designates an electric motor, which generates a steering assisting force, which connected between the output terminals of a motor driving circuit 110 comprising a bridge circuit of four FETs 110a to 110d, and which is controllably driven when a drive unit 120 turns on one of the opposed pairs of FETs in the motor driving circuit 110. In such a typical electrically powered steering system, when the FET 110c, for instance, causes a short circuit, a closed circuit is formed between the FET 110c, the motor, a parasitic diode in the FET 110d and the FET 110c. When a drive operates the steering wheel, the motor 100 is rotated by a steering shaft through a speed reducer to induce an electromotive force in the motor 100. Since a large current flows in the closed circuit, the closed circuit works as a braking circuit to require the driver to apply an excessive steering force, which deteriorates the driving operation.
When the motor is reversely rotated in such a state that the FET 110c has caused short circuit as stated earlier, no braking action is provided since no current flows because of the absence of the closed circuit. This means that the braking force, or the required steering force, becomes different, depending on the steering directions. Since the steering force in the right direction and that in the left direction become unbalanced, the steering feeling extremely deteriorates.
In order to cope with this problem, there has been provided a proposal as disclosed in JP-B-796387. Now, a conventional electrically powered steering system will be explained, referring to FIG. 11. In this Figure, parts identical to those shown in FIG. 10 are designated by the same reference numerals, and explanation of these parts will be omitted.
As shown in FIG. 11, the motor 100 is connected to the output terminals of the motor driving circuit 110 through a relay 130.
When the FET 110c causes a short circuit as stated earlier, this conventional powered steering system opens the relay 130 to prevent a closed circuit or braking circuit from being provided, preventing the steering feeling from being deteriorated.
Although this conventional electrically powered steering system can open the relay 130 in case of failure to prevent the braking circuit from being provided, the electrically powered steering system has created a problem in that the behavior of the vehicle becomes unstable under the influence of the moment of inertia of the motor 100 since the motor 100 is coupled to the steering shaft through the speed reducer.
It is an object of the present invention to solve this problem, and to provide an electrically powered steering system capable of making the behavior of a vehicle stable to provide good steering feeling even in case of failure.
According to a first aspect of the present invention, there is provided an electrically powered steering system comprising an electric motor for assisting a steering force based on a steering torque, and an electric motor driving circuit for driving the electric motor based on an inertia compensation current in case of failure, wherein the inertia compensation current becomes greater as the electric motor has an angular acceleration increased and provides the electric motor with a torque in a direction identical to the electric motor angular acceleration.
According to a second aspect of the present invention, there is provided an electrically powered steering system comprising an electric motor for assisting a steering force based on a steering torque, and an electric motor driving circuit for driving the electric motor based on a viscosity compensation current in case of failure, wherein the viscosity compensation current becomes greater as the electric motor has an angular speed increased and provides the electric motor with a torque in a direction opposite to the electric motor angular speed.
According to a third aspect of the present invention, there is provided an electrically powered steering system comprising an electric motor for assisting a steering force based on a steering torque, and an electric motor driving circuit for driving the electric motor based on an inertia compensation current and a viscosity compensation current in case of failure, wherein the inertia compensation current becomes greater as the electric motor has an angular acceleration increased and provides the electric motor with a torque in a direction identical to the electric motor angular acceleration, and wherein the viscosity compensation current becomes greater as the electric motor has an angular speed increased and provides the electric motor with a torque in a direction opposite to the electric motor angular speed.
According to a fourth aspect of the present invention, the electric motor is prohibited from operating when failure occurs at the electric motor or the circuit.
According to a fifth aspect of the present invention, the system further comprises a vehicle speed sensor for detecting a vehicle speed, wherein the compensation current is modified in response to the detected vehicle speed.
According to a sixth aspect of the present invention, the compensation current becomes greater as the vehicle speed increases.
According to a seventh aspect of the present invention, the compensation current is determined based on a preset vehicle speed in case of failure at the vehicle speed sensor.
According to an eighth aspect of the present invention, there is provided an electrically powered steering system comprising an electric motor for assisting a steering force based on a steering torque, an electric motor driving circuit for driving the electric motor, the circuit comprising a bridge circuit including four switching devices, and the circuit having the motor connected between output terminals thereof through a resistor, and a switch for short-circuiting both ends of the resistor in normal operation.
According to a ninth aspect of the present invention, the system further comprises a vehicle speed sensor for detecting a vehicle speed, wherein the resistor has a value of resistivity modified in response to the detected vehicle speed.
According to a tenth aspect of the present invention, the value of resistivity becomes smaller as the vehicle speed increases.
According to an eleventh aspect of the present invention, the value of resistivity is determined based on a preset vehicle speed in case of failure at the vehicle speed sensor.
According to a twelfth aspect of the present invention, there is provided an electrically powered steering system comprising an electric motor for assisting a steering force based on a steering torque, and a switch for short-circuiting both terminals of the electric motor, wherein the switch is driven by a duty signal in case of failure.
According to a thirteenth aspect of the present invention, the system further comprises a vehicle speed sensor for detecting a vehicle speed, wherein the duty signal for driving the switch has a duty ratio modified in response to the detected vehicle speed.
According to a fourteenth aspect of the present invention, the duty ratio becomes greater as the vehicle speed increases.
According to a fifth aspect of the present invention, the duty ratio is determined based on a preset vehicle speed in case of failure at the vehicle speed sensor.
According to a sixteenth aspect of the present invention, the system further comprises an alarm for warning that failure occurs in case of failure.
In accordance with the first aspect, an adverse effect by the moment of inertia of the motor can be compensated even in the case of failure to make the behavior of the vehicle stable and improve steering feeling.
In accordance with the second aspect, an adverse effect on steering feeling by the viscosity of the motor can be compensated even in case of failure to make the behavior of the vehicle stable, improving steering feeling.
In accordance with the third aspect, adverse effects by the moment of inertia and by the viscosity of the motor can be compensated even in case of failure to make the behavior of the vehicle stable, improving steering feeling.
In accordance with the fourth aspect, when the continuation of the operation of the motor can lead to burning because of failure, the operation of the motor can be stopped to provide the electrically powered steering system with further improved safety.
In accordance with the fifth aspect, the degree of compensation in the inertia or the viscosity can be modified in response to the detected vehicle speed to further improve steering feeling.
In accordance with the sixth aspect, the viscosity or the inertia can be more greatly compensated in a high speed region to make the behavior of the vehicle stable, further improving steering feeling.
In accordance with the seventh aspect, sufficient compensation can be provided even in case of failure, improving steering feeling.
In accordance with the eighth aspect, when the system is in normal operation, the resistor can be short-circuited to prevent the driving circuit from being adversely affected. When failure occurs in the system, the resistor can work as a braking resistor to give a braking force to the motor, preventing the behavior of the vehicle from deteriorating because of the moment of inertia of the motor, and improving steering feeling even in case of failure.
In accordance with the ninth aspect, the braking force, which is applied to the motor, can be modified in response to the detected vehicle speed to further improve steering feeling.
In accordance with the tenth aspect, the system can apply a greater braking force to the motor in a high speed region to make the behavior of the vehicle stable, further improving steering feeling.
In accordance with the eleventh aspect, a sufficient braking force can be applied even in case of failure, improving steering feeling.
In accordance with the twelfth aspect, when failure occurs in the system, a braking force can be applied to the motor so as to prevent the behavior of the vehicle from deteriorating because of the moment of inertia, improving steering feeling even in case of failure, and offering an advantage in that the system can be made smaller and economical because of no need for an additional part, such as a resistor for braking.
In accordance with the thirteenth aspect, the braking force, which is applied to the motor, can be modified in response to the detected vehicle speed to further improve steering feeling.
In accordance with the fourteenth aspect, the system can apply a greater braking force to the motor in a high speed region to make the behavior of the vehicle stable, further improving steering feeling.
In accordance with the fifth aspect, a sufficient braking force can be applied even in case of failure, improving steering feeling.
In accordance with the sixteenth aspect, in case of failure, a warning can be provided to the driver, providing the electrically powered steering system with further improved safety.